_Mallein_ is a substance analogous to tuberculin, and is made by growing a pure culture of _Bacillus mallei_ in glycerine-veal broth in flat flasks, with free access of calcined air. After a month's growth the culture is sterilised, filtered, concentrated, and mixed with an equal volume of a .5 per cent. solution of carbolic. The dose is 1 cc., and it is used, like tuberculin, for diagnostic purposes. If the suspected animal reacts to the injection, it is suffering from glanders. Reaction is judged by three signs, namely, a rise of temperature 2-3° C., a large "soup-plate" swelling at the site of inoculation, and an enlargement of the lymphatic glands. _Swine fever_, _foot-and-mouth disease_, _chicken cholera_, _dysentery_, _rinderpest_, and other diseases of animals have microorganisms intimately related to them. There is a group of diseases due to the presence in the blood or tissues of _hæmatozoa_, that is, protozoa which can live and perform their function in the blood. Amongst these are malaria, sleeping sickness, and other tropical diseases in man, and surra and various hæmatozoa in horses, fish, frogs, or rats. _Malaria._ Although a _Bacillus malariæ_ has been described as the cause of this disease, it is now almost universally supposed that the true cause is a protozoan parasite. In 1880 Laveran first described this organism, and the discovery was confirmed by Marchiafava, Celli, and others. Laveran claimed that it occurred in four different forms during the progress of its life-history: (_a_) _Spherical or Irregular Bodies_ attached to the blood corpuscle, or free in the blood plasma. They are a little smaller than the blood-cells, and may or may not contain pigment. They eventually invade the corpuscles, possess more pigment, and lose their amœboid movement. Within the red blood corpuscles they increase in size until they reach the adult stage. (_b_) _Segmentation Forms_, often assuming a rosette shape, follow next. They are pigmented, are possibly a sporing stage, and are finally set free in the blood. (_c_) _The Crescents_, or _Semilunar Bodies_, are free in the blood, but motionless. They are colourless, have a distinct membrane, and generally show a little pigment about the middle; they taper towards the poles. They appear in the blood after the fever has existed for some time, occurring chiefly, sometimes only, in the quotidian and malignant types of malaria. (_d_) _The Flagellated Bodies_ apparently occur only in the blood outside the body. They are extracorpuscular bodies, and possess several long flagella, and are therefore actively motile. They are derived from the crescents or irregular intracorpuscular bodies.